docs: def missing_kegg_mapping(self):
          """list: List of genes with no mapping to KEGG."""
          kegg_missing = []
          for g in self.genes:
              keggs = g.protein.filter_sequences(KEGGProp)
              no_sequence_file_available = True
              for k in keggs:
                  if k.sequence_file:
                      no_sequence_file_available = False
                      break
              if no_sequence_file_available:
                  kegg_missing.append(g.id)
          return list(set(kegg_missing))